The present invention relates to thermoplastic sheet and film products. More particularly, the invention relates to a rotatable perforated molding element or screen for use in the preparation of embossed and/or perforated thermoplastic sheet or film and a method for connecting the molding element to the supporting roll or drum.
Perforated and embossed thermoplastic sheet or film have many useful applications. Perforated film is used in gardening and farming to prevent the growth of grass and other weeds while permitting more moisture to be transmitted through the film to the soil beneath. Perforated film is also used for making disposable baby diapers. U.S. Pat. No. 3,814,101 discloses diapers employing perforated thermoplastic film which permits the flow of liquid in the direction of the absorbent material in the diaper but substantially reduces the possibility of flow in the opposite direction. Embossed film without perforations are also used in making baby diapers, and in other applications where it is important that the film have a cloth-like appearance or feel.
One of the methods for perforating thermoplastic sheet or film is disclosed in U.S. Pat. No. 3,054,148, issued to Zimmerli, which is hereby incorporated by reference. The Zimmerli patent discloses a stationary drum having a molding element mounted around the outer surface of the drum and being adapted to rotate freely thereon. A vacuum chamber is employed beneath the screen or molding element to create a pressure differential between the respective surfaces of the thermoplastic sheet to cause the plasticized sheet to flow into the perforations provided in the molding element and thereby cause a series of holes to be formed in the sheet.
Difficulties are frequently encountered when placing the molding element or screen onto the drum, particularly in achieving a snug fit about the drum and replacing a torn or broken screen rapidly. When the perforations in the molding element or screen are very fine, it is necessary to connect the screen to the drum by using a laser to weld the edges of the screen together after the screen is wrapped about the drum. However, such welding in general cannot be done in the field. Furthermore, the welds are commonly deficient in strength when they must be made while the screen is wrapped about the drum.
Difficulty is also commonly encountered in forming a one-piece screen which will have the exact dimensions desired. It is commonly the practice to cut a series of strips from a screen and weld panels of the screen together to form one large rectangular screen which can be wrapped about the drum and welded along one seam to achieve a fit upon the drum. However, welding a series of strips together is time consuming and expensive. It is also extremely difficult to get a snug fit of the screen to the outside of the drum. Furthermore, the screen tends to loosen on the drum with use.